The present invention relates to a neutron ionization chamber of a gamma ray compensation type, and more particularly, to an improvement for a compensating characteristic of ionization currents caused by incident gamma rays.
In a conventional ionization chamber of this type, a cylindrical compensating electrode is concentrically disposed within a cylindrical signal electrode, and a cylindrical high voltage electrode is concentrically disposed outside the signal electrode. These three electrodes are held in a metallic casing having a gas-tight structure. The inner and outer circumferential surfaces of the high voltage and signal electrodes are respectively coated by a material including enriched boron in which the percentage of boron 10 is greater than in natural boron, in which boron 10 constitutes about 19%. Boron 10 has a large neutron reaction cross section and thus has a high neutron reaction rate. Ring-shaped insulating materials are disposed between the casing and the high voltage electrode, the high volage and signal electrodes, and between the signal and compensating electrodes to electrically insulate and concentrically retain them. A gas for ionization is sealed within the casing to produce ionization between the respective electrodes.
In the ionization chamber constituted as above, when a radiant ray, i.e., a gamma ray enters the ionization chamber, the gas between the compensating and signal electrodes is ionized by secondary electrons which are caused to be emitted from the electrode surfaces by the incident gamma rays. A part of the gamma rays causes ionization of the gas by directly acting on it, but such ionization is insignificant.
Accordingly, when a voltage negative to the voltage of the signal electrode is applied to the compensating electrode, an ionization current I.sub..gamma. flows from the signal electrode to the compensating electrode. Between the signal and high voltage electrodes, the incident gamma rays also cause ionization of the gas as well between the signal and compensating electrodes, and boron 10 atoms of the enriched boron on the inner and outer circumferential surfaces of the high voltage and signal electrodes, respectively, react with neutrons and emit charged particles at a high speed, also causing ionization of the gas.
Accordingly, when a voltage positive to the voltage of the signal electrode is applied to the high voltage electrode, a neutron ionization current In, caused by neutrons, and a gamma ray ionizatin current I.sub..gamma.' flow from the high voltage electrode to the signal electrode in proportion to the intensities of neutron and gamma rays. Hence, a signal current of Is=In+I.sub..gamma.' -I.sub..gamma. flows in the signal electrode.
The values I.sub..gamma. and I.sub..gamma.' of the ionization currents caused by the gamma rays are proportional to the surface areas of the electrodes, the rate at which the secondary electrons are emitted from the electrode surfaces, and the number of gas molecules between the electrodes. These values can be adjusted such that approximately I.sub..gamma.' =I.sub..gamma. by suitably choosing the electrode diameters. In such a condition, the signal current Is equals the neutron current In, and thus is proportional to a neutron flux.
As mentioned above, an object of the ionization chamber is to compensate or cancel the gamma ray ionization currents and take out only the neutron ionization current In.
The rate at which secondary electrons are emitted from the electrode surfaces depends on the electrode material, and on the energy of the incident gamma rays, and the manner of the dependence also depends on the electrode materials. Accordingly, even when the gamma ray ionization currents caused by the incident gamma rays are completely compensated at some energy level of the gamma rays, the rate of the secondary electrons emitted from the respective electrodes is changed by an energy change of the incident gamma rays, since the electrode surfaces of the neutron sensing ionization chamber portion are coated with a material containing enriched boron and are different from the electrode surfaces of the gamma ray-sensing ionization chamber portion without a coating material. Thus, a difference between the gamma ray ionization currents I.sub..gamma. and I.sub..gamma.' is produced, and these ionization currents are not completely compensated so that the neutron ionization current in proportional to a neutron flux cannot be exactly taken out.